Rainbow Electronics U3741BM - Manual (Page 5)

5
U3741BM
4662B–RKE–10/04
To determine f
LO
, the construction of the IF filter must be considered at this point. The
nominal IF frequency is f
IF
= 1 MHz. To achieve a good accuracy of the filter’s corner fre-
quencies, the filter is tuned by the crystal frequency f
XTO
. This means that there is a
fixed relation between f
IF
and f
LO
that depends on the logic level at pin mode. This is
described by the following formulas:
The relation is designed to achieve the nominal IF frequency of f
IF
= 1 MHz for most
applications. For applications where f
RF
= 315 MHz, the MODE must be set to ‘0’. In the
case of f
RF
= 433.92 MHz, the MODE must be set to ‘1’. For other RF frequencies, f
IF
is
not equal to 1 MHz. f
IF
is then dependent on the logical level at pin MODE and on f
RF
.
Table 1 summarizes the different conditions.
The RF input either from an antenna or from a generator must be transformed to the RF
input pin LNA_IN. The input impedance of that pin is provided in the electrical parame-
ters. The parasitic board inductances and capacitances also influence the input
matching. The RF receiver U3741BM exhibits its highest sensitivity at the best sig-
nal-to-noise ratio in the LNA. Hence, noise matching is the best choice for designing the
transformation network.
A good practice when designing the network is to start with power matching. From that
starting point, the values of the components can be varied to some extent to achieve the
best sensitivity.
If a SAW is implemented into the input network, a mirror frequency suppression of
∆
P
Ref
= 40 dB can be achieved. There are SAWs available that exhibit a notch at
∆
f = 2 MHz. These SAWs work best for an intermediate frequency of IF = 1 MHz. The
selectivity of the receiver is also improved by using a SAW. In typical automotive appli-
cations, a SAW is used.
Figure 3 on page 6 shows a typical input matching network for f
RF
= 315 MHz and
f
RF
= 433.92 MHz using a SAW. Figure 4 on page 6 illustrates an input matching to 50
Ω
without a SAW. The input matching networks shown in Figure 4 are the reference net-
works for the parameters given in the “Electrical Characteristics”.
MODE 0 (USA) f
IF
f
LO
314
---------- = =
MODE 0 (Europe) f
IF
f
LO
432.92
------------------ = =
Table 1. Calculation of LO and IF Frequency
Conditions Local Oscillator Frequency Intermediate Frequency
f
RF
= 315 MHz, MODE = 0 f
LO
= 314 MHz f
IF
= 1 MHz
f
RF
= 433.92 MHz, MODE = 1 f
LO
= 432.92 MHz f
IF
= 1 MHz
300 MHz < f
RF
< 365 MHz, MODE = 0
365 MHz < f
RF
< 450 MHz, MODE = 1
f
LO
f
RF
1
1
314
---------- +
------------------- = f
IF
f
LO
314
---------- =
f
LO
f
RF
1
1
432.92
------------------ +
---------------------------- = f
IF
f
LO
432.92
------------------ =